In recent years, significant developments in laser technology have led to its application in the field of ophthalmic surgery. In particular, laser surgery has become the technique of choice for ophthalmic surgical applications. In certain ophthalmic laser procedures, surgeons use a mechanical device termed a microkeratome to cut a layer of the anterior surface of the cornea in order to expose the underlying corneal stroma to which the laser is applied. However, complications surrounding the use of the microkeratome and its metal blade have resulted in research into improved techniques that are performed exclusively by a laser system. Such all-laser techniques obviate the need for mechanical devices pre- or post-operatively, and provide significantly improved precision.
Despite these advances in laser technology, the use of such systems for ophthalmic surgical procedures remains fraught with substantial mechanical limitations, particularly in the area of developing a stable interface between an incident laser beam and the eye of a patient. Ophthalmic surgery is a precision operation and requires a very precise coupling between the surgical tool (i.e., the laser beam) and the region to be disturbed (i.e., a portion of the patient's eye). Even a very small movement of the eye with respect to the intended focal point of the laser beam can not only lead to non-optimal results, but might even result in permanent damage to non-renewable tissue within the eye, leading to precisely the opposite result than that desired. Given that eye movement is often the result of autonomic reflex, it should be understood that there must be some means of stabilizing the position of a patient's eye with respect to an incident laser beam in order to avoid the intolerable consequence of relative movement.
Heretofore, the major technique used to compensate for relative eye motion with respect to an incident laser beam has been to have the patient focus on a stationary target. This involves providing a visual target to the eye undergoing surgery, and requiring that the patient retain focused on the perceived target feature. While this technique has provided some small benefit, it places all of the burden of minimizing relative motion upon the patient, and does not allow for any gross autonomic reflex motions, e.g., as when the patient might be startled. In this technique, the target provides optical interface, while the patient's conscious responses provide the feedback mechanism.
An additional technique involves the use of an optical eye tracking apparatus, whereby a selected eye feature is targeted for monitoring by an optical device, and as the targeted feature displaces as the result of eye movement, its displacement is characterized and fed into the incident laser beam control apparatus as a compensation signal. This second technique offers a substantial improvement over the first, particularly when it is implemented in addition to a patient-driven target focusing mechanism. However, such systems are inordinately expensive since a second, completely independent optical path must be provided between a patient's eye and a surgical apparatus in order to accommodate the eye tracking apparatus. Further expense and complexity are incurred when it is considered that an eye tracking apparatus requires an additional software component in order to be operative, which software component must be integrated into a laser delivery system. Considerations of interoperability must be met as well as the provision for an automatic shutdown of the laser system in the event of the loss of target feature lock.
Accordingly, a simple mechanical system, if properly designed, is able to best meet the imperatives of interfacing a laser delivery system with a target object. If the goal is to minimize relative analog motion, an analog stabilization device would necessarily offer the most advantageous solution.
In this regard, certain mechanical stabilization devices have been proposed, particularly, a corneal applanation device which is the subject of U.S. patent application Ser. No. 09/172,819, filed Oct. 15, 1998 and commonly owned by the assignee of the present invention, the entire contents of which are expressly incorporated herein by reference. Such a mechanical device directly couples a patient's eye to the laser's delivery system being affixed to both the laser and the anterior surface of a patient's cornea. The corneal coupling, in these devices, is typically implemented by lowering an applanation fixture over the anterior surface of the cornea under pressure. It is assumed in these forms of devices that pressure applied normal to the corneal surface will restrict conventional motion of the cornea thereby stabilizing the eye along a major access normal to the device.
However, although this assumption may hold true in a large number of cases, it certainly does not have universal application. Moreover, in the cases where it does hold, the device/cornea interface should be established with the iris centered, for best results. The actual establishment of an effective device/corneal interface is an exercise in trial-and-error, resulting in a great deal of frustration to doctor and patient, as well as considerable eye fatigue. For ophthalmic laser procedures where eye tissue is to be photodisrupted, it is extremely important for the laser beam to be properly focused to a specific focal spot in the tissue that is to be effected. Not only is it extremely important to have good focal definition, but also for the focal point to have the proper dimensionality (i.e., the correct spot diameter and shape). In order to accommodate this, it is necessary for the laser beam to be as free from aberrations as possible. In particular, for ophthalmic laser procedures involving the cornea, it happens that the spherical geometry of the cornea introduces optical aberrations as a result of its shape, which are separate and distinct from aberrations introduced by the laser's own optical system. Significantly, these corneal induced aberrations distort the definition of the focal spot of a laser beam as the beam is focused to a position within corneal tissue.
Due to the spherical geometry of the anterior surface of the cornea, two specific types of aberrations are of particular importance with regard to beam distortion; spherical aberration (which relates to points on the optical axis of the laser beam) and coma (which relates to points that are off-axis). Spherical aberration and coma are similar to one another in that they both arise from a failure to image or focus optical ray traces onto the same point. Spherical aberration relates to a distortion that can be characterized as radial in nature, with some radial directions being stretched while other radial directions are shrunk, converting thereby, an ideally circular spot into an elliptical spot. Coma distortion, on the other hand, implies an elongation along one radius in the shape of a circle, resulting in a “cometlike” shape. Accordingly, any structure which interfaces between a curved, anterior surface of the cornea and laser delivery system must be applanatic in nature. By definition, an applanatic lens is one that is free from both spherical aberration and coma.
As is recognized by the present invention, applanatic refraction at the anterior surface of the cornea can be effectively accomplished by flattening the anterior surface. With such a corneal reconfiguration, the beam will be free of aberrations (other than chromatic) which would otherwise result from an interface with the cornea's native spherical anterior surface.
Because of the foregoing considerations, a simple mechanical interface device was developed which is the subject of copending parent application Ser. No. 09/772,539. That device is able to stabilize the eye against relative motion with respect to a laser beam used for ophthalmic surgical procedures without relying on secondary mechanical considerations, such as surface tension, friction, or the like. Such a device should be able to present an optical feature to an incident laser beam in a stable, well-characterized location, such that the beam is able to interact with the feature without regard to opto/electronic feedback mechanisms. In addition to maintaining a proper orientation between the eye and a laser delivery system during ophthalmic laser surgery, such a device should applanate the eye during surgery while reducing inter-ocular pressure during the surgical procedure. Such a device should be easy for a clinician to affix, as well as being simple and cost effective to manufacture and use.
Aside from structural differences, applanation lenses are unlike many other types of lenses used in lens systems such as those described in U.S. Pat. Nos. 5,359,373 and 6,142,630, because an applanation lens is not part of an expensive lens system and can easily be removed from mechanical interface devices. As such applanation lenses used in the past have been disposable. Because of cost considerations they have been formed of a polymer materials.
The preferred method of sterilization for such lenses is by gamma sterilization. Gamma sterilization is relatively inexpensive and does not leave a residue, as some sterilizing gases tend to do. However, it has been found that gamma radiation at levels sufficient for sterilization (e.g., 25 kGy-40 kGy) causes problems because it discolors polymers and certain glasses, or otherwise lowers the transmittance of light through these materials. Transmittance as used herein refers to optical efficiency or the ability of a material to transmit light.
Because the dose of radiation may be variable between sterilization lots, the amount of transmittance loss is not uniform. Thus, sterilization of these materials using gamma radiation introduces an uncontrolled variable into the system. This is a serious problem because of the need to be able to focus the laser beam at precise locations in or on the cornea, and to consistently deliver, a predetermined level of energy to that location.
The variability caused by sterilization has the effect of significantly reducing the predictability from one surgical procedure to another, and adds to the time of a surgical procedure because the system must be recalibrated for each use. The loss of transmittance also requires greater power depending on the reduction in the lens's ability to transmit light.
Thus, there is also a need for an applanation lens that remains stable when gamma radiation is applied and does not discolor or lose transmittance below 90% for wavelengths of light from 275 nm to 2500 nm, particularly at about 1053 nm which is the near infrared wavelength of the femtosecond laser that is preferably used in the system in this application.
Because the applanation lens is in direct contact with the cornea of the eye, it must be composed of a material that is biocompatible with corneal tissue. In addition, the applanation lens material must be able to withstand the applied laser energy without melting, oxidizing, or creating byproducts that are not compatible with corneal tissue.